1. Technical Field
The present invention relates to an apparatus for detecting a physical quantity, and in particular, to an apparatus for concurrently detecting a plurality of types of physical quantities, such as acceleration.
2. Related Art
A wide variety of types of detection apparatuses have been known for detecting physical quantities, such as acceleration and pressure. Such detection apparatuses are inevitable for various industrial products including vehicles and industrial robots.
One type of such a physical quantity detecting apparatus is disclosed, for example, by Japanese Patent Laid-open publication Nos. 8-145717 and 2000-221054. These references provide a capacitor type of detection apparatus equipped with a sensor element, a C-V (electric capacitance-voltage) conversion circuit, and a signal processing circuit. The sensor element has capacitance changing in response to mechanical energy such as acceleration and pressure. The C-V conversion circuit is placed to covert voltage depending on changes in the capacitance of the sensor element to allow the voltage to have corresponding values. The signal processing circuit is placed to sample/hold voltage outputted from the C-V conversion circuit. The outputs from the signal processing circuit are subjected to detection of the physical quantities such as acceleration and pressure.
It is frequently desired to detect acceleration generated in different plural directions (for the sake of simplifying the explanation, such directions are to be X-axis and Y-axis directions of the orthogonal coordinate system). To meet such a demand, the above conventional detecting apparatus can be modified into an apparatus capable of detecting, by itself, acceleration generated in the two or more directions.
It is required for such an apparatus to have two sets of circuitry. One set is directed to acceleration generated in the X-axis direction and the other is directed to acceleration generated in the Y-axis direction. To be specific, the one set is composed of X-axis directional components comprising a sensor element whose capacitance depends on the X-axis directional acceleration, a C-V conversion circuit, and a signal processing circuit. The other set is composed of Y-axis directional components comprising a sensor element whose capacitance depends on the Y-axis directional acceleration, a C-V conversion circuit, and a signal processing circuit.
There is a drawback with this configuration, though. This drawback will appear when both the X-axis and Y-axis directional signal processing circuits are brought into synchronous sampling operations. That is, it is difficult for the signal processing circuits to hold the voltage values with precision, because there are various influences resulting from factors including parasitic capacitance which may be present among wiring patterns. Such influences, if actually occurs, will lead to the problem that changes in the capacitance at each sensor element cannot be detected with precision.
This problem will be described in more detail. In general, a sample and hold circuit uses a capacitor charged to hold (memorize) voltage. If a large amount of parasitic capacitance exists between output signal lines from both the X-axis directional C-V conversion circuits, the voltage outputted from the Y-axis directional C-V conversion circuit fluctuates when the X-axis directional signal processing circuit applies sampling to the voltage outputted from the X-axis directional C-V conversion circuit. As a result, the Y-axis directional signal processing circuit is obliged to sample the fluctuating voltage. At the same time, it is also true that, when the Y-axis directional signal processing circuit applies sampling to the voltage outputted from the Y-axis directional C-V conversion circuit, the output voltage of the X-axis directional C-V conversion circuit causes fluctuations. Hence the X-axis directional signal processing circuit has no choice but to sample the fluctuated voltage. These sampling operations give errors to the voltages sampled by the respective signal processing circuits.
Occurrence of this kind of erroneous sampling operation is not limited to the configuration in which a capacitance type of sensor is used, but may be possible even when other types of sensor are employed. On top of this, it is considered that such a drawback becomes remarkable when a plurality of capacitance type sensor elements are formed on the same semiconductor substrate.
This formation will lead to a configuration where the output terminals of both the X-axis and Y-axis directional sensor elements are mutually connected through a parasitic capacitance on the same semiconductor substrate. Hence both of a first signal line from the X-axis directional sensor element to the X-axis directional C-V conversion circuit and a second signal line from the Y-axis directional sensor element to the Y-axis directional C-V conversion circuit are mutually connected via the parasitic capacitance on the semiconductor substrate. The sampling operation carried out by one signal processing circuit therefore has a large influence on that carried out by the other signal processing circuit.